1. Field of the Invention
The present invention relates to the structure of an electroluminescent (in this specification, referred to as organic EL) device for use as a display device in an information terminal apparatus, such as a computer, a mobile phone, or a television, and also relates to a manufacturing method thereof.
2. Background of the Related Art
Recently, there has been an increasing trend toward portability of information terminals, and development of power saving displays, which are necessary to enhance portability. Among displays, organic EL displays in particular have drawn attention, and the development thereof is at the stage of pursuing practical applications.
In order to realize power saving displays, it is believed that it is most effective to drive an organic EL element by an active element, such as a thin-film transistor (TFT). The reason for this is that, when an active element is used, an organic EL element can be driven by a DC voltage, and can be driven at a low voltage without imposing a load thereon in the range of high luminous efficiency. In the case of simple matrix driving, which uses no active devices, the luminance must be increased by applying a high voltage during a selection period. Accordingly, a significant load is imposed on the organic EL element, and in addition, the luminance efficiency is decreased. As a result, the life thereof is naturally shortened.
An active matrix method using a TFT or the like is a strong candidate for use in a power saving organic EL display; however, it has the shortcoming that the aperture ratio, which indicates the ratio of the light-emitting area to the display area for display, is decreased. When the aperture ratio is decreased, luminance in each pixel must be enhanced in order to increase the display luminance. In the case described above, a driving voltage is increased, power consumption is thereby increased, and hence, a problem may arise in that the life is shortened due to the load imposed on the organic EL element. In order to solve the problem described above, as shown in FIG. 2, in the structure of an organic EL element (device), a transparent cathode can be formed in order to emit light from a side opposite to the substrate (IEEE, TRANSACTIONS ON ELECTRON DEVICES, VOL. 44, NO.8, PP 1188-1203). In particular, an anode 5, a hole injection layer 4, a light-emitting layer 3, a cathode 2, and a transparent auxiliary cathode 21 are sequentially formed on a substrate 1 in this order, and light is emitted from the transparent auxiliary cathode 21 side. In the structure described above, the light may not be emitted from the TFT substrate side; however, the light transmittance of the cathode used in the structure described above is approximately one-half of that of the substrate, and as a result, a problem may arise in that the brightness of the display is actually reduced.